Sometimes -- particularly with letterbooks -- we may have item-level metadata for some portions of the book, and these items stand on their own as intellectual items.
Often the letters span multiple digital files, or portions of them, which brings up numbering issues. Do you number the intellectual item, or do you number the page?

We decided it's best to do both.
In order to be able to deliver the book as a book, and the letters as separate items, we digitize the book, noting which pages become what items later.
To do this, we use File:BookToItems.